The invention is directed to a method for the transmission of digital signals in burst mode, that is, in what is referred to as time-division multiple access (TDMA).
In addition to wireless transmission of television and/or radio signals by radiotelegraphy, these signals are often transmitted from a central to a plurality of subscribers via coaxial cables. Due to the transmission properties of the coaxial cables, the transmission thereby usually ensues with carrier frequency methods, whereby quadrature amplitude modulation (QAM) methods are provided for achieving a corresponding bandwidth efficiency given digital transmission. In this method, two carrier signals that are orthogonal to one another in terms of phase are transmitted, whereby each carrier can be modulated with a plurality of amplitude levels. For demodulation, an optimally phase-exact carrier signal must also be available in the receiver in order to keep losses due to crosstalk of the respectively orthogonal channel low.
A greater bandwidth for the transmission from the central to the subscribers is required given expansion of such coaxial cable transmission networks by a greater number of programs and by what are referred to as interactive services such as, for example, interactive television in the form of teleshopping and multimedia applications, so that only a few frequency bands are still available for the transmission of the signals of the interactive services from the subscribers to the central. Difficulties occur in providing adequate bandwidth available for these signals. Additional difficulties result in that only a short time span in a periodic cycle is available to the individual subscriber stations because of the use of a shared transmission path to the central. The signals deriving from the subscribers are transmitted in what is referred to as time-division multiple access (TDMA). At the beginning of the reception time, however, an optimally phase-exact synchronization to the received carrier signal must ensue in the receiver. The problem thus arises that only a short synchronization time and, thus, a short response time are available for the phase synchronization circuits that are usually employed. However, this corresponds to a large bandwidth, so that a corresponding phase precision of the demodulated carrier signal can only be achieved with difficulty or with great outlay correspondingly long signal bursts and, thus, a lower number of subscribers.